Medical implants often utilize communication systems in order to transmit information between the medical implants and external devices. Communication systems in medical implants demand very low power consumption due to battery supply limitations. To reduce overall energy consumption, the exchange of data should be performed efficiently. In turn, efficient packet exchange between the medical implant and the external device requires minimization of the time required to setup the receiver prior to demodulating data. This is especially important for communication systems with small amounts of data to transfer where, in these cases, the radio setup time can be a significant proportion of the overall communication time.
Conflicting with the need for small receiver setup times, modern receivers require several adjustments in order to optimally receive data. These adjustments include gain adjustments, compensation for frequency differences between receiver and transmitter local oscillators, and determining bit timing for data demodulation. To facilitate radio receiver setup, a signal called a preamble is often placed in a data packet prior to the actual data in the payload. The preamble can be used to provide information to acquire frequency, gain settings and timing and setup the overall radio prior to receiving the payload.
Medical implants along with other small and low power applications use simple quartz crystal based oscillators that are not temperature compensated. Larger, higher power and accurate temperature controlled crystal oscillators are not capable of being used. This results in a relatively inaccurate frequency reference that is relative large (+/−20 kHz) and approaches the typical data rates (50-200 kbps). Consequently local oscillators used in transmitters and receivers in low power applications have inaccuracies that need to be handled through the use of automatic frequency control systems.
Prior to receiving the payload, radio receivers in a communication system must typically perform the following operations: (i) adjust the carrier frequency to match the incoming carrier frequency given the relatively large frequency offsets, (ii) acquire timing information to optimally sample the received signal in the center of each bit, (iii) reliably identify the beginning of the packet for synchronization purposes, (iv) adjust amplifier gains to avoid saturating filter stages and optimize signal levels into analog to digital converters, and (v) remove DC offsets which is especially important in direct conversion architectures. These setup operations can occupy a significant proportion of the overall communication time when the amount of data to be transferred is small, thereby also consuming a significant amount of power.